Stator bars are known to include a plurality of interwoven conductive copper strands defining a conductive element (e.g., the so-called green bar) having a rectangular shape (plain Roebel bars).
This bar is covered with insulation which can be made of a mica tape impregnated with a resin (insulated Roebel bar), for example.
Because of the rectangular shape, four sharp corners are defined, which during operation generate very high electrical field peaks. These peaks limit the maximum electric voltage that can be applied to the stator bars and, in turn, limit the achievable power.
In order to reduce the electrical field peaks, it is known to round the corners of the conductive element, such that a radius between 0.5-2.5 millimeters is achieved.
The beneficial effect of the rounding is quite high when the radius is small, but when increasing the radius the beneficial effect decreases. For a radius of about 2.5 millimeters, the beneficial effect cannot be practically further improved in this way.
In order to reduce the electrical field peaks at the corners of the stator bars, WO 2007/139,490 discloses to vary the insulation permittivity in the circumferential direction, such that the insulation has a higher permittivity at its corners.
Since the permittivity is only regulated in the circumferential direction, an insulation such as that disclosed in WO 2007/139,490 does not efficiently use the dielectric material constituting it.
In addition, DE 198 11 370 discloses to vary the insulation permittivity in the radial direction.
With reference to the insulation permittivity, model calculations showed that when the ratio∈(corners)/∈(elsewhere) between the insulation permittivity at the corners (close to the conductive element) and the insulation permittivity elsewhere (in the radial direction) is greater than an optimum value, an electric field is induced at the transition that may be higher than the electric field at the corners (close to the conductive element) with insulation having a uniform permittivity (i.e., a single permittivity value).
It is clear that in this case no real improvement is achieved, since a very high electric field actually exists in the insulation.